The 'rubber' bag
This picture explain better how the "rubber" bag is connected to the board. This connection has to be done is a way as not to puncture the very thin polyethylene. This connection takes a lot of care. Therefore it was further developed to get it done with less risk for such punctures.
The picture shows the wooden valve tube that is inserted in the middle of the wooden board. The end of the valve stem just sticks out of this valve tube.
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